1. Field Of The Invention
The present invention relates to a cation exchange membrane having high durability. More particularly, the present invention is concerned with a cation exchange membrane for use in electrolysis of an alkali metal chloride, which comprises a quadrilateral base membrane and, disposed on at least one marginal area of one surface of the base membrane, a diffusion coating having a capability for an alkali metal hydroxide to diffuse therethrough at a diffusion velocity greater than that at which the alkali metal hydroxide diffuses through a fluorocarbon polymer layer constituting the base membrane when the base membrane is composed of one fluorocarbon polymer layer or through one outermost layer of the base membrane which layer has thereon the diffusion coating and has a water sorption smaller than that of the opposite outermost layer of the base membrane, when the base membrane is composed of a plurality of fluorocarbon polymer layers.
The cation exchange membrane of the present invention is free, for a prolonged period of time, from the local lowering of the mechanical strength of the membrane and the local damaging of the membrane attributed thereto, which are caused by the precipitation of an alkali metal chloride within and the formation of oxygen in nascent state within the base membrane during the electrolysis of an alkali metal chloride as described later. By the use of the cation exchange membrane of the present invention, an alkali metal chloride can be stably electrolyzed for a prolonged period of time.
2. Discussion of Related Art
It is generally desired for a cation exchange membrane that is to be used in the electrolysis of an alkali metal chloride to exhibit a high current efficiency and a low electric resistance and have an excellent mechanical strength.
The electrolysis of an alkali metal chloride is performed in a cation exchange membrane-containing electrolytic cell, the most popular of which is a filter press-type electrolytic cell having a quadrilateral shape in vertical cross-section. The filter press-type electrolytic cell comprises a number of unit cells each comprising a unit cell frame, an anode, an anode compartment, a cathode, a cathode compartment and a cation exchange membrane situated to separate the two compartments and fastened through gasket frames between the unit cell frames. With respect to the detailed structure of the filter press-type electrolytic cell, reference is made to, for example, U.S. Pat. Nos. 108,742 and 4,111,779.